Insulated electric heater and method of making the same



March 17, 1942. H, D. HOLLER I 2,276,973

INSULATED ELECTRIC HEATER AND METHOD OF MAKING THE SAME Filed Sept. 15, 1939 kg, N 10 WITNESSEZ: INVENTOR lib/4e! [fa/Z22:

Patented Mar. 17, 1942 INSULATED ELECTRIC HEATER AND METHOD OF MAKING THE SAME Homer D. Holler, Export, Pa., aasignor to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania 1 Application September 15, 1939, Serial No. 295,053

7 Claim.

This invention relates to insulated electric heaters and to the method of insulating suc heaters.

In producing insulated electric heaters, it has been found to be diiiicult to provide an insulation of uniform homogeneous composition which is capable of withstanding the high temperatures sometimes encountered in service while having the characteristic of being non-corrosive. It has also been diiilcult to insulate the heater elements, theexisting methods requiring considerable care in practicing the steps thereof, while the results are not always uniform.

An object of this invention is the provision of an insulated electric heater unit, the insulation of which is a dense homogeneous mixture of materials which are non-corrosive and have high electrical resistivity.

A further object of this invention is the provision of a method of insulating electric resistor wires of the heater type, whereby a hard, dense, non-corrosive insulating coating isformedabout the wire. 4

Other objects of this invention will become apparent from the following description when taken in conjunction with the accompanying drawing, in which Figure 1 isa side view of a resistor element embodying the teachings of this invention; and

Fig. 2 is an enlarged view in section of the element of Fig. 1, taken along the line II---II. In practicing this invention, any of the wellknown metallic resistor units may be employed, and particularly the metallic heating wires it, as illustrated in the drawing, such as are utilized. in the heating units of electric roosters, strip heaters, hot plates and the like, All of these elements should be so insulated as to prevent oxidation thereof and to electrically insulate them from the apparatus in which they are employed, while permitting the heat developed therein to fiow therefrom.

In order to satisfactorily insulate such heating wires, this invention provides for utilizing the reaction products of a mixture of materials as the insulating material i! for embedding the heating wire i therein as illustrated in the drawing. The materials employed for forming the insulating material comprise a mixture of magnesium oxide and ammonium borate. In making the mixture for applying to the wire to be insulated, the magnesium oxide and the ammonium borate are thoroughly mixed, with the ammonium borate constituting from to 50% by weight of the mixture, after which sufllcient water is added to the resulting mixture to give it a creamy, mud-like consistency so that it can be poured. The amount of water employed to give the mud-like consistency to the mixture will vary depending upon the grade of magnesium oxide employed, but inall cases should be'so limited to that quantity which gives the mud-like consistency to the mixture. Where an extremely pure grade of magnesium oxide is utilizedfit is preferred to make the mixture in a 1:1 ratio by weight of magnesium oxide and ammonium borate. For the impure grades of magnesium oxide, smaller quantities within the range of 25%, to 50% by weight of the ammonium borate can be employed giving a mixture having a ratio ranging between 1:1 and 2:1 by weight of the magnesium oxide to the ammonium borate.

With the magnesium oxide and ammonium borate formed into the mixtur having a mudlike consistency, it is applied to the wire to be insulated in any of the well known manners for completely embedding the wire. Thus the mixture can either be poured directly about the wire or it can be extruded about the wire, or films of the mixture can be applied to both sides of a heating element to envelop it. Regardless of the method employed for enveloping or embedding the heating wire in the mixture of magnesium oxide and ammonium borate, it is found that where the mixture, as applied, has a mudlike consistency it will set about the heating wire or unit in a manner similar to the setting of plaster of Paris. In this form, the embedded heating wire can be moved without danger to the applied coating.

In order to produce the finished insulating coating about the wire, the embedded article is placed in any suitable furnace and subjected to a baking treatment at a temperature of not, less than (3., and preferably between 120 C. and C., it being found that such baking effects a reaction upon the components of the mixture constituting the coating. This reaction between the magnesium oxide and ammonium borate is accompanied by the evolution of ammonia and moisture from the mixture, and gives a resulting product comprising a uniform, intimate and homogeneous mixture of magnesium oxide and magnesium borate.

In a particular embodiment of this invention,

a material which is inert with respect to the reaction between the magnesium oxide and ammonium borate is included in the mixture to add strength to the resulting insulating coating and to reduce the cost per pound for insulation. Potin the chemical reaction between the mesh lzl. and 2:1 oif moriiuru borate whether or is present in the l I ammonium borate, magucs oxide ters flint are employed in 112:3 ratio.

When the flint, magnesium. oxide or; mouiuin borate is mixed with water to I" mud-like consistency sud applied to and treated in the warmer P re do scribed, it is found that the no part gnesium oxideand ammonium borate but iu the resulting insulating costing, uniformly distributed therein and aids in giving added strength to the reaction products of the magnesium oxide and ammonium borste, the resulting insulating coating consisting of a uniform, and homogeneous mixture of flint, maguesium oxide and magnesium hora-rte.

Upon examination, it is found that the in sulating coating comprising the urifiorm and homogeneous mixture of magnesimo oxide, mu magnesium box-ate, either by itseli or with the potters flint as herelnbeiore descri ed, is quite dense and extremely hard. Further, it is found that the resulting reaction product good thermal conductivity, an extremely high elem trlcal resistivity and the desired characteristic of being non-corrosive. in addition to the foregoing desired characteristics, the ulstiou will not crack when subjected to the extreme changes in temperature as are cncountred irl bursting the iormeduult'as a heating element, nor is it affected by the humidity of the atmosphere in which it is operated, the insulating coating being free of decrepitation.

Although this invention has been described with reference to a particular embodiment thcreof, it is, 01' course, not to be limited thereto ex= cept insofar as is necessitated by the scope oi the appended claims.

I claim as my invention:

l. The method of insulating electric resistor wires of the heater type comprising the steps oi, embedding the wire in a mixture of magnesium oxide and ammonium borate having a ratio of between 1:1 and 2:1 and sufficient water to give the mixture :1 mud-like consistency, letting the mixture set in position about the wire, and heating the mixture while in position about the wire to effect a reaction between the magnesium oxide and ammonium borate whereby ammonia is evolved and a hard, dense, noncorrosive coating of intimately mixed magnesium oxide and magnesium borate is formed about the wire.

2. The method of insulating electric resistor e wire in a mixture oi mug .esiui simonium boraie barbie a ratio .1: cud 32:1 and sulilcieu'l; water u mud-dike consistency, in.

sitiou about the wire, and We while in position about the are of st least C. to dri J ,esctiori between the magnesium:- oxide u borate whereby ammonia is cvo :2. hard, dense, non-corios1ve coating oi mutely mixed magnesium oxide and magncsiujze bursts is formed about the wire.

, All insulated electric heater comprisin n combination, a resistor wire and a. coating of is sulating material disposed thereon, said co, ing comprising a hard, dense, non-corrosive lam terlsl having a high electrical resistivity comprising an intimate and homogeneous l. ture of magnesium oxide and magnesium as essential binding elements thereof.

4. An insulated electric heater comprising combination, a. resistor wire and a coating insulating material disposed thereon, said lug comprising arr-intimate and homogeu mixture or magnesium oxide and magnesium rate formed by effecting a reaction between hesium oxide and ammonium borate in the or between 1:1 and 2:1, the coating being u and non-corrosive.

5. An insulated electric heater comprislzo combination, a resistor wire and a coat insulating material disposed thereon, said c ing comprising a hard, dense, non-corrosive serial having a high electrical resistivity sisting of an intimate and homogeneous oi potters flint, magnesium oxide and 1 s5 nesium borate, the magnesium oxide and r uesium borate being present as essential clcmenm in the coating and binding the no" mogeneously distributed potters flint.

6. An insulated electric heater compri" combination, a resistor wire and a cos. of insulating material disposed thereon, said coat lug comprising an intimate and homogeneous mixture of potters flint, magnesium oxide and magnesium borate, the magnesium oxide and magnesium borate having been formed by e'llfecting a reaction between magnesium oxide ammonium borate in the ratio of between 1:1 and 2:1, the coating being dense and non-corlOSlVe.

7. An insulated electric heater comprising in combination, a resistor wire and a coating of insulating material disposed thereon, said coating comprising an intimate and homogeneous mixture 01 .potters flint, magnesium oxide and magnesium borate, the magnesium oxide magnesium borate having been formed by etfectlng a reaction between magnesium oxide and ammonium borate in the presence of the potters flint, the ratio of the ammonium borate, mag nesium oxide and potters flint being 1:2:3, coating being dense and non-corrosive.

HOMER D. HOLLEB. 

